1) Field of the Invention
Semiconductor oxidation and diffusion processing and low pressure CVD of silicon dioxide (doped and undoped), silicon (doped and undoped), single -and poly-crystalline, silicon, silicon nitride, and other thin films (conductive, semi-conductive, and insulating) on semiconductor wafer substrates. Low pressure chemical vapor deposition of silicon dioxide (doped and undoped), polysilicon (doped and undoped), silicon nitride, and other thin films (conductive, semi-conductive, and insulating) or larger for display panel substrates.
2) Description of the Prior Art
In semiconductor integrate circuit manufacturing, phosphorus-doped oxides are used to dope underlying silicon layers. These doped silicon layers are used to form semiconductor devices. In order to increase yields of these semiconductor devices, the doped layers must have the maximum uniformity achievable.
One method of forming phosphorus doped oxides (also called P-glass, phosphosilicate glass, and PSG) is by deposition in a hot walled furnace. As shown in FIG. 1, wafers 16 are placed on a sled 20. The sleds 20 typically sit on a carrier 18 which is used to insert and remove the sleds 20 from the furnace tube 10. Reactant gasses enter the furnace at the source end 24 of the furnace tube and are exhausted on the other end through an exhaust 14.
One process for forming phosphorus doped oxide is: EQU 4 POCL.sub.3 +3O.sub.2 .fwdarw.2P.sub.2 O.sub.5 +6CL.sub.2
The reactant gasses (POCL.sub.3 +O.sub.2) flow into the tube 10 at the source end 24 pass the wafers on which the phosphosilicate glass (PSG) is formed.
When attempting to diffuse a large number of wafers at the same time with phosphorous and particularly wafers of a substantially large diameter, there must be substantially uniform diffusion of the phosphorous into each of the wafers to obtain the desired resistivity of the doped layer. Thus, if the wafers that are farthest from the source end (gas inlet end) do not receive substantially the same diffusion of phosphorous as the wafers closest to the source end there will be a very substantial difference in both the thickness of the doped glass and the concentration of the phosphorous in the wafers so as to lower the yield of the wafers.
Deposition reactions are complicated and depend on many factors such as furnace temperature, gas flow rates, wafer spacings, etc. One major factor in uniformly doping wafers, the reactant gasses must be well mixed and turbulent. This mixing will ensure that the reactants will uniformly dope within the wafers from edge to center, top to bottom and from wafer to wafer (front to back of the carrier).
One method of increasing phosphorus doping uniformity is to increase the spacing between the wafers to improve the gas flow between the wafers. However, increasing the spacing decreases the wafer throughput thus increasing manufacturing costs. Another method of increasing the uniformity is to place dummy wafers in front of product wafers in a carrier. While this improves the film uniformity, more uniformity is still required. Moreover, the dummy wafers take up space where product wafers could be thus reducing wafer throughput and increasing costs.
Therefore, It is desirable to have a device or method of improving the deposition uniformity (e.g., thickness and resistivity) of closely spaced wafers without decreasing throughput and increasing costs.